Apparatus and method for catalytic treatment of exhaust gases

ABSTRACT

An exhaust gas treatment apparatus ( 20 ) for reducing the concentration of NO x , HC and CO in an exhaust gas stream ( 18 ) such as produced by a gas turbine engine ( 12 ) of a power generating station ( 10 ). The treatment apparatus includes a multifunction catalytic element ( 26 ) having an upstream reducing-only portion ( 28 ) and a downstream reducing-plus-oxidizing portion ( 30 ) that is located downstream of an ammonia injection apparatus ( 24 ). The selective catalytic reduction (SCR) of NO x  is promoted in the upstream portion of the catalytic element by the injection of ammonia in excess of the stoichiometric concentration, with the resulting ammonia slip being oxidized in the downstream portion of the catalytic element. Any additional NO x  generated by the oxidation of the ammonia is further reduced in the downstream portion before being passed to the atmosphere ( 22 ).

FIELD OF THE INVENTION

This invention relates generally to the treatment of a gas streamcontaining nitrogen oxides, for example but not limited to the gasesproduced by the combustion of a fossil fuel, wherein the gas stream mayalso contain hydrocarbons, carbon monoxide and/or ammonia.

BACKGROUND OF THE INVENTION

The control of undesirable emissions such as oxides of nitrogen(NO_(x)), hydrocarbons (HC) including volatile organic compounds (VOC),and carbon monoxide (CO) that are generated by power producers such asautomobiles and electrical power generating stations is a well-studiedfield. The Background section of U.S. Pat. No. 5,891,409 provides auseful summary of the conditions and chemistries that produce suchemissions and the approaches used to limit the release of thesepollutants to the environment.

One technology for the control of oxides of nitrogen that is currentlybeing used commercially at large land-based electrical power generatingstations is selective catalytic reduction (SCR). The flue gases from apower station have a net oxidizing effect due to the high proportion ofoxygen that is provided to ensure adequate combustion of the hydrocarbonfuel. Thus, the oxides of nitrogen that are present in the flue gas canbe reduced to nitrogen and water only with great difficulty. Thisproblem is solved by selective catalytic reduction wherein the flue gasis mixed with anhydrous ammonia and is passed over a suitable reductioncatalyst at temperatures between about 150-550° C., and preferablybetween 300-550° C., prior to being released into the atmosphere. Theammonia is not a natural part of the combustion exhaust stream, butrather, it is injected into the exhaust stream upstream of the catalystelement for the specific purpose of supporting one or more of thefollowing reduction reactions:4NH₃+4NO+O₂→4N₂+6H₂O  (1)4NH₃+2NO+2NO₂→4N₂+6H₂O  (2)8NH₃+6NO₂→7N₂+12H₂O  (3)4NH₃+4NO+O₂→4N₂+6H₂O  (4)Reducing agents other than ammonia, such as for example hydrazine,methyl hydrazine, monomethyl amine, and urea, or mixtures thereof, ormixtures thereof with ammonia, may also be employed in the processesdescribed herein.

It is also known to combine an SCR process with a catalytic oxidizingprocess to treat an exhaust gas flow by oxidizing carbon monoxide tocarbon dioxide and by oxidizing hydrocarbons to carbon dioxide andwater. The oxidizing process is typically located upstream of theammonia injection location and upstream of the reducing catalyst becausethe oxidizing catalyst will also function to oxidize ammonia, which isundesirable when it decreases the amount of ammonia available forreduction of the NO_(x) and because it produces additional NO_(x)compounds. U.S. Pat. No. 5,589,142 describes an emissions abatementsystem where an emission steam is passed sequentially through a firstoxidizing catalyst, an ammonia injection location, a reducing catalyst,and then a second oxidizing catalyst. In this process, the amount ofammonia that is injected is controlled as a function of the NO_(x)concentration and is specifically limited to a stoichiometric value.Thus, no excess ammonia is present in the emission stream as it leavesthe reducing catalyst and there is no concern about generatingadditional NO_(x) compounds in the trailing second oxidizing catalyst.

Modern air quality regulations mandate continuingly reduced emissionlevels for power generating plants, while at the same time fuelefficiency requirements continue to increase. Combustion controls alonemay prove inadequate to satisfy these often-conflicting goals, and thuscontinued the improvement of post-combustion exhaust gas treatmentsystems is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in following description in view of thefollowing drawings:

FIG. 1 is a schematic illustration of a power generating plantincorporating an improved exhaust gas treatment system.

FIG. 2 is a table comparing the performance of a reducing only catalyticprocess with a reducing-plus-oxidizing catalytic process.

FIG. 3 is a table summarizing exemplary results of catalyst testing at anatural gas fired pilot plant with a reducing-plus-oxidizing catalyticprocess.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have developed a process for the post-combustioncatalytic treatment of exhaust gases that is useful in power generationapplications to reduce the concentrations of oxides of nitrogen,hydrocarbons (including oxygenated hydrocarbons, such as for examplealdehydes) and carbon monoxide to sub-2 ppm levels, while at the sametime limiting the amount of ammonia injection that is necessary tosupport the incorporated SCR process and also maintaining ammonia slipto the atmosphere to below 2 ppm.

FIG. 1 is a schematic illustration of one embodiment of the presentinvention for a power generating plant 10 including a gas turbine engine12 wherein energy is produced by the combustion of a hydrocarbon fuel 14in air 16 to produce shaft power and resulting in a flow of exhaustgases 18 containing undesirably high concentrations of NO_(x),hydrocarbon and CO pollutants. The power generating plant 10 includes apost-combustion treatment apparatus 20 for reducing the concentration ofthe pollutants prior to the release of the exhaust gas to the atmosphere22. The treatment apparatus 20 incorporates an ammonia injectionapparatus 24 and a multifunction catalytic element 26 located downstreamof the ammonia injection apparatus 24 relative to the flow of exhaustgases 18. The catalytic element 26 includes a reducing-only portion 28and a combined reducing-plus-oxidizing portion 30 located downstream ofthe reducing-only portion 14.

During operation of the power plant 10, the ammonia injection apparatus24 and reducing-only portion 28 of the catalytic element 26 willfunction together for the selective catalytic reduction of the oxides ofnitrogen present in the flow of exhaust gases 18, such as described byEquations 1 through 4 above. The SCR catalyst may be any catalyticmaterial designed to facilitate a reaction between NH₃ and NO_(x) suchas to form N₂. Catalysts may include but are not limited to V/TiO₂ basedmaterials and zeolite based materials. The catalyst material(s) selectedfor the reducing-only portion 28 may be any appropriate material knownin the art, such as titanium dioxide and at least one oxide of tungsten,vanadium, molybdenum, silicon, aluminum, iron, titania-zirconia,magnesium, manganese or yttrium, or their mixtures, for example. Otheradditives, such as for example sulfate, lanthanum, barium, zirconium,may also be present. One such composition is titanium oxide with 1-5%V₂O₅, 0-10% WO₃, and 0-10% MoO₃. The percentages expressed herein areweight percentages unless otherwise indicated. Zeolite materials includeacidified forms of zeolite ZSM-5, zeolite beta, mortenite, andfaujasite; promoted with small amounts of base metals, such as forexample iron, cobalt and nickel.

The reducing-plus-oxidizing portion 30 contains material(s) that supportthe reduction reactions of Equations 1 through 4 as well as material(s)that support one or more of the following oxidizing reactions:CO+O₂→CO₂  (5)C_(a)H_(b)O_(y)+(a+b/4−y/2)O₂ →aCO₂ +b/2H₂O  (6)4NH₃+7O₂→4NO₂+6H₂O  (7)4NH₃+5O₂→4NO+6H₂O  (8)2NH₃+2O₂→N₂O+3H₂O  (9)4NH₃+3O₂→2N₂+6H₂O  (10)The reducing-plus-oxidizing portion 30 preferably supports reaction 10in favor of reactions 7 through 9, as will be discussed further below,in order to minimize the production of additional quantities of NO_(x).Thus, the oxidizing catalyst material is able to decompose NH₃ toprimarily N₂. Typical oxidizing catalyst materials include transitionmetals of groups 6B, 8B and 9, and preferably include copper, platinum,palladium, chrome, iron, nickel, rhodium, gold, silver, ruthenium andmixtures thereof, although the present invention is not limited to anyparticular oxidizing or reducing catalytic material. Any known form ofcatalyst structure may be used, such as pellets, granules, cylinders ora monolithic structure. The reducing-only portion 28 and thereducing-plus-oxidizing portion 30 may be formed as a monolithicstructure, or the two portions may be formed separately.

The post combustion treatment apparatus 20 allows the selectivecatalytic reduction process to be operated with an excess of ammoniathat is beyond the stoichiometric concentration. This is advantageousbecause it has been observed that the efficiency of the reductionreaction decreases as the level of NO_(x) removal exceeds 90+%, therebymaking it difficult to achieve sub-2 ppm NO_(x) levels by injecting onlya stoichiometric quantity of ammonia. A ratio of NH₃:NO_(x) of about1.05:1 to about 2:1, or preferably about 1.05:1 to about 1.2:1 may beused in certain embodiments to achieve the desired level of NO_(x)reduction. Such excess ammonia levels would normally result in ammoniaslip of greater than about 5-15 ppm in prior art SCR processes, and anyuse of a downstream oxidizing catalyst to decrease the concentration ofammonia in the slip would result in an increase in the NO_(x) levelsusing prior art systems due to the oxidation of NH₃ to NO_(x). Thepresent inventors have solved this dilemma by providing a multifunctioncatalytic element 26 that includes both reducing and oxidizing functionsat its downstream portion 30. This novel concept allows the excessammonia to be decomposed after completion of the primary NO_(x)reduction function, while at the same time providing a continuingreduction capability for further reacting with the additional NO_(x)that is produced as a result of the oxidation of the excess ammonia.

In addition to decomposing the excess ammonia introduced into theexhaust gas stream 18, the oxidizing function of thereducing-plus-oxidizing portion 30 also serves to decrease CO and HCpollutants to a desired low level. Prior art systems that oxidize CO andHC pollutants upstream of the SCR process create a need for theinjection of additional amounts of ammonia because the oxidation processalso converts up to 65% of the NO that passes through the oxidationcatalyst into NO₂, and it is known that a higher molar ratio of ammoniais required to reduce NO₂ than to reduce NO in a downstream SCR process(equation 3). Advantageously, the present treatment apparatus 20 avoidsthis problem by performing the oxidation process downstream of the SCRprocess. Thus, the present invention decreases the amount of ammoniathat is needed to reduce the NO_(x) that is produced by the gas turbineengine 12 when compared to a prior art process having an oxidizingcatalyst followed by an SCR process.

Advantageously, the selectivity of the oxidizing catalyst to producewater and nitrogen from ammonia (equation 10) rather than producingwater and an oxide of nitrogen (equations 7-9) is higher in the presenceof the reduction catalyst than it is when the oxidizing catalyst isfunctioning alone. It is known that NO is a main product of thecatalytic oxidizing of ammonia on a traditional CO catalyst thatcontains platinum or palladium. These metals are commonly used for COand VOC oxidation in the flue gases from power generation stations. Thecombination of an oxidizing catalyst (for example Pt/Pd) with a reducingcatalyst dramatically increases the selectivity of the ammoniadecomposition process toward nitrogen formation. As a result, ammoniareduction during this improved process is not accompanied by elevatedNOx emissions when compared with the SCR-only process, as illustrated inthe comparison table of FIG. 2. Thus, the synergy provided by thereducing-plus-oxidizing portion 30 of the multifunction catalyst 26serves to further reduce the levels of NO_(x) released to the atmosphere22.

Optionally, a low level of oxidizing catalyst function (e.g. lower thanin the reducing-plus-oxidizing portion) may be provided upstream of themultifunction catalyst element 26. This oxidizing function may beprovided as a discrete oxidizing catalyst 32, or by impregnating theupstream portion 28 of the multifunction catalyst element 26 with asmall amount of oxidizing material, for example between 0.001-3 wt. % ofthe catalyst or preferably between 0.01-1 wt. % of the catalyst. Thisconcentration is comparatively lower than between 0.01-15 wt. % of thecatalyst or preferably between 0.1-5 wt. % of the catalyst that may beused in the downstream reducing-plus-oxidizing portion. Thisconfiguration will enhance the CO and hydrocarbon oxidation activity ofthe process, but will require that the process be operated at a slightlygreater NH₃ to NO_(x) ratio, as a small portion of the ammonia will bedecomposed prior to being involved in the reducing reactions.

The types, volumes and structure of the catalytic materials of themultifunction catalytic element 26 may vary depending upon therequirements of a particular application. The reducing catalyst materialmay be identical between the reducing-only portion 28 and thereducing-plus-oxidizing portion 30, or they may be different materials.The reducing-only portion of the multi-function catalyst may be in therange of 10-90% of the total catalyst volume, with one embodiment havingthe reducing-only portion being 60% of the total catalyst volume and thereducing-plus-oxidizing portion being 40% of the total catalyst volume.

Exemplary results of catalyst testing at a natural gas fired pilot plantare illustrated in the table of FIG. 3. The catalyst used was amonolithic catalyst with a density of 200 cells per square inch (CPSI).The catalyst dimensions were 150 mm by 150 mm by 300 mm. The NH₃/NOxmolar ratio was 1.05-1.15. The split of the total catalyst volumebetween reducing-only and reducing-plus-oxidizing portions was either60%/40% or 50%/50% respectively, as indicated in the figure. Thereduction-only catalyst included 1.7 wt. % of vanadium/TiO₂ and thereduction-plus-oxidizing catalyst included a reduction catalyst having1.7 wt % of vanadium/TiO₂ impregnated with 2.8 g/ft³ each of platinumand palladium.

Several examples of the advantageous performance of the presentinvention are discussed in the following paragraphs; however, it isfirst be instructive to examine the performance of a prior artreducing-only catalyst. For such comparisons, consider a 5% V/TiO₂wash-coated monolith exposed to a process stream comprising 410 ppmtoluene, 10% O₂, 2.5% H₂O, with the balance being N₂ at a gas hourlyspace velocity (GHSV) of 18,000. At a temperature of 380° C., theconversion of toluene through this catalyst is less than 5%. For thesame reducing catalyst exposed to a process stream comprising 410 ppmCO, 10% O₂, 2.5% H₂O, and the balance N₂ at a GHSV of 18,000, theconversion of CO at a temperature of 380° C. is less than 5%. Finally,for the same catalyst exposed to a process stream comprising 100 ppmNH₃, 10% O₂, 2.5% H₂O, and the balance N₂ at a GHSV of 18,000, at atemperature of 370° C., the conversion of NH₃ is 36%, at 350° C., theconversion of NH₃ is 20%, and at 330° C. the conversion of NH₃ is lessthan 10%.

Considering now an embodiment of the present invention, a layered bedcatalyst configuration consisting of a reducing-only portion and areducing-plus-oxidizing portion was evaluated for its ability to treatNO_(x) containing emissions streams. The wash-coated monolithic catalystemployed in the reducing-plus-oxidizing portion (50%) consisted of 0.1%Pt/0.1% Pd impregnated onto an SCR catalyst, where the SCR catalystconsisted of 2.7% V/2% FeSO₄/TiO₂ in both portions. The catalyst wasexposed to a process stream consisting of 30 ppm NO_(x), between 30 and45 ppm NH₃, 10% O², 3.5% H₂O, balance N₂ at 305° C. and a GHSV of20,000. The Table 1 below reports the NO_(x) reduction efficiency andNH₃ slip for varying NH₃ feed concentrations. The ammonia conversionrefers to the conversion of the excess NH₃; that is to say theconversion of the fraction of ammonia that is not converted to N₂ byreactions involving NO_(x).

TABLE 1 NH₃ Feed NOx Reduction Conversion of Concentration EfficiencyExcess NH₃ 45 ppm 99.0% 84.6% 40 ppm 98.9% 87.5% 35 ppm 97.2% 96.7% 30ppm 91.2%  >99%These results demonstrate that the process/apparatus described herein iscapable of operating over a wide range of NH₃ to NO_(x) ratios.

The catalyst configuration described above was evaluated for its abilityto reduce NOx emissions over temperatures between 370° C. and 230° C.The catalyst configuration was exposed to a process stream consisting of30 ppm NO, 40 ppm NH₃, 10% O², 3.5% H₂O, balance N₂ at a GHSV of 20,000.Table 2 reports the NO_(x) reduction efficiency and NH₃ conversion as afunction of temperature.

TABLE 2 Temperature, NOx Reduction Conversion of ° C. Efficiency ExcessNH₃ 372° C. 93% 98% 355° C. 94% 99% 338° C. 96% 99% 321° C. 98% 99% 304°C. 99% 88% 286° C. 99% 66% 270° C. 99% 19% 255° C. 99% 11% 230° C. 99% 7%

The catalyst configuration described above was evaluated for its abilityto decompose 93 ppm acetaldehyde at a GHSV of 20,000 in a process streamcomprising 10% O₂, 3.5% H₂O balance N₂. Results are presented in Table3.

TABLE 3 Temperature, ° C. Acetaldehyde Conversion, % 370° C. >99% 330°C. >99% 310° C. >99% 290° C. >99% 260° C. 80.2% 

The catalyst configuration described above was evaluated for its abilityto decompose 160 ppm toluene at a GHSV of 20,000 in a process streamcomprising 10% O₂, 3.5% H₂O balance N₂. Results are presented in Table4.

TABLE 4 Temperature, ° C. Toluene Conversion, % 300° C. 99.4% 290° C.99.3% 275° C. 98.8% 260° C. 96.9% 245° C. 87.3%

The catalyst configuration described above was evaluated for its abilityto decompose 500 ppm CO at a GHSV of 20,000 in a process streamcomprising 10% O₂, 3.5% H₂O balance N₂. Results are presented in Table5.

TABLE 5 Temperature, ° C. CO Conversion, % 255° C. 99.2% 235° C. 98.3%215° C. 97.7% 190° C. 97.4% 165° C. 96.1% 145° C. 92.0% 125° C. 77.5%For CO, toluene and acetaldehyde, carbon dioxide was the onlycarbon-containing reaction product detected in the effluent stream.

In a further example, a wash-coated reducing-plus-oxidizing monolith(200 cells/in²) comprising 1% Pd/3% Cu/0.1% Pt/5% V/TiO₂ was evaluatedfor its ability to decompose NH3 with minimal NO_(x) formation. Thecatalyst was exposed to a process stream comprising 100 ppm NH₃, 10% O²,2.5% H₂O, and the balance N₂ at a GHSV of 18,000. The ammonia conversionand concentration of NOx in the process effluent stream is reported inTable 6.

TABLE 6 Temperature [NO_(x)], ppm NH₃ Conversion 340° C. 3.74 ppm 99.2%320° C. 1.16 ppm 95.1% 295° C. 0.48 ppm 47.5%These results demonstrate the ability of the multifunctionreducing-plus-oxidizing catalyst to decompose NH₃ with minimal NO_(x)formation.

In a further example, a wash-coated monolith (200 cells/in²) comprising3% Cr/3% Cu/0.2% Pt/5% V/TiO₂ was evaluated for its ability to decomposeNH₃ with minimal NO_(x) formation. The catalyst was exposed to a processstream comprising 100 ppm NH₃, 10% O₂, 2.5% H₂O, and the balance N₂ at aGHSV of 18,000. The ammonia conversion and concentration of NOx in theprocess stream are reported in Table 7.

TABLE 7 Temperature [NO_(x)], ppm NH₃ Conversion 365° C. <0.5 ppm >99%335° C. <0.5 ppm >99% 310° C. <0.5 ppm >99% 290° C. <0.5 ppm >99%These results further demonstrate the ability of the multifunctionreducing-plus-oxidizing catalyst to decompose NH₃ with minimal NO_(x)formation.

In a final example, a monolithic layered bed catalyst configurationconsisting of a 2.5% V/TiO₂ inlet layer upstream of a 3.5% Cu/0.7%Pd/2.5% V/TiO₂ outlet layer was exposed to an emissions streamconsisting of 300 ppm NO, 350 ppm NH₃ at 330° C. and a GHSV of 9,000.The inlet layer and outlet layer were each 50% of the total catalystvolume. At these process conditions, the NOx reduction efficiency was95.8%, and the NH₃ reduction efficiency, calculated based on the amountof excess ammonia, was 78.2%.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the appended claims.

1. A method of treating a stream of gases containing oxides of nitrogen,carbon monoxide and hydrocarbons, the method comprising: injectingammonia into the gas stream to beyond a stoichiometric concentrationwith the oxides of nitrogen; then reducing oxides of nitrogen by passingthe gas stream over a reducing catalyst; and then oxidizing carbonmonoxide, hydrocarbons and ammonia while simultaneously further reducingoxides of nitrogen by passing the gas stream over areducing-plus-oxidizing catalyst.
 2. The method of claim 1, furthercomprising injecting the ammonia in a quantity sufficient to achieve amolar ratio of ammonia to oxides of nitrogen to between 1.05:1 and1.2:1.
 3. The method of claim 1, further comprising providing thereducing catalyst and the reducing-plus-oxidizing catalyst as amonolithic multifunction catalytic element comprising an upstreamreducing-only portion and a downstream reducing-plus-oxidizing portion.4. The method of claim 1, further comprising beginning to oxidize carbonmonoxide, hydrocarbons and to decompose ammonia prior to the reducingstep by passing the gas stream over an oxidizing catalyst that providesfor a lower level of oxidizing activity than does thereducing-plus-oxidizing catalyst.
 5. The method of claim 1, furthercomprising beginning to oxidize carbon monoxide, hydrocarbons and todecompose ammonia simultaneously with the reducing step by impregnatingthe reducing catalyst with a concentration of oxidizing catalystmaterial that is less than a concentration of oxidizing catalystmaterial contained in the reducing-plus-oxidizing catalyst.
 6. A methodof treating an exhaust stream to reduce a concentration of oxides ofnitrogen in the exhaust stream, the method comprising: injecting areducing agent into the exhaust stream to beyond a stoichiometricconcentration with the oxides of nitrogen; and bringing the reducingagent-containing exhaust stream into contact with a multifunctioncatalytic element comprising an upstream reducing-only portion and adownstream reducing-plus-oxidizing portion.
 7. The method of claim 6,wherein the step of injection comprises injecting ammonia in a quantitysufficient to achieve a molar ratio of ammonia to oxides of nitrogen ofbetween 1.05:1 and 1.2:1.
 8. The method of claim 6, further comprisingproviding the upstream reducing-only portion and the downstreamreducing-plus-oxidizing portion on a common monolithic substrate.
 9. Themethod of claim 6, further comprising providing a reducing catalystmaterial in the downstream reducing-plus-oxidizing portion that isdifferent than a reducing catalyst material of the upstreamreducing-only portion.
 10. A method of treating an exhaust stream toreduce a concentration of oxides of nitrogen, carbon monoxide andhydrocarbons in the exhaust stream, the method comprising: injectingammonia into the exhaust stream; and bringing the ammonia-containingexhaust stream into contact with a multifunction catalytic elementcomprising an upstream portion comprising a first reducing catalystmaterial and a first concentration of an oxidizing catalyst material anda downstream portion comprising a second reducing catalyst material anda second concentration of an oxidizing catalyst material, the secondconcentration being greater than the first concentration.
 11. The methodof claim 10, further comprising injecting the ammonia to beyond astoichiometric concentration with the oxides of nitrogen.
 12. The methodof claim 10, wherein the first reducing catalyst material and the secondreducing catalyst material comprise the same material.
 13. The method ofclaim 10, wherein the upstream portion and the downstream portion areformed as a monolithic structure.
 14. The method of claim 10, furthercomprising forming the upstream portion to comprises 10-90% of themultifunction catalytic element volume.
 15. A power generating apparatuscomprising: a gas turbine engine for combusting a fuel in air to produceshaft power and a flow of exhaust gases comprising oxides of nitrogen,carbon monoxide and hydrocarbons; a treatment apparatus for receivingthe exhaust gases prior to passing the exhaust gases to atmosphere, thetreatment apparatus comprising: an ammonia injection apparatus forinjecting ammonia into the exhaust gases to beyond a stoichiometricconcentration with the oxides of nitrogen; and a multi-functioncatalytic element disposed downstream of the ammonia injection apparatusand comprising an upstream reducing portion and a downstreamreducing-plus-oxidizing portion.
 16. The apparatus of claim 15, whereinthe reducing portion and the reducing-plus-oxidizing portion are formedas a monolithic structure.
 17. The apparatus of claim 15, furthercomprising a reducing catalyst material of the reducing portion being amaterial different that a reducing catalyst material of thereducing-plus-oxidizing portion.
 18. The apparatus of claim 15, furthercomprising: the ammonia injecting apparatus selected to inject theammonia in a quantity sufficient to achieve a molar ratio of ammonia tooxides of nitrogen in the exhaust stream to between 1.05:1 and 1.2:1;and the multi-function catalytic element effective to limit the ammoniaslip to below 2 ppm.
 19. The apparatus of claim 15, wherein the reducingportion of the multi-function catalytic element comprises 10-90% of atotal volume of the catalytic element.
 20. The apparatus of claim 15,further comprising an oxidizing catalyst function disposed upstream ofthe reducing-plus-oxidizing portion.
 21. The apparatus of claim 20,wherein the oxidizing catalyst function comprises an oxidizing catalystmaterial impregnated into the upstream reducing portion of themulti-function catalytic element.
 22. The apparatus of claim 20, whereinthe oxidizing catalyst function comprises an oxidizing catalyst elementdisposed upstream of the multi-function catalytic element.